The color of each pixel on an Advanced Graphing plot is the average of all the colors graphed there, including the background. In the default "Light" scheme, this makes the primary colors look like a washed-out pastel:

If you want, say, the blue to look BLUE, the trick is to repeat the function two or more times, like this:

If you combine that tip with Eddie's tip above, you can even get not just gray with RGB(0,0,0), but BLACK: